The effects of electromagnetic waves leaking from electronic devices on digital devices has come to be a problem in recent years with the rapid advance of the use of digital technology in such electronic devices. In addition, not only have electromagnetic waves conventionally been known to cause erroneous operation by having an effect on electric signals flowing through electronic circuits, but there are also concerns regarding their effect on the body, and electromagnetic shielding materials have been developed as a countermeasure against this.
In addition, simultaneous to being required to demonstrate a high level of electromagnetic wave shielding (electromagnetic wave shielding properties), materials that have flexibility and bending resistance, are thin and are lightweight are desired for use as electromagnetic shielding materials.
Although electromagnetic shielding materials that use woven fabrics or non-woven fabrics are used as materials that satisfy these requirements, these fabrics have the problem of causing a decrease in electromagnetic wave shielding properties unless the weight per unit area and thickness of the woven fabric or non-woven fabric per se are increased and the amount of metal coating is also increased.
Means for solving this problem are disclosed in Patent Document 1 and Patent Document 2 indicated below in which adhesion of a metal coating and plating processability can be improved with a smaller increase in the thickness of the fabric after forming a metal coating in comparison with the prior art by using an electromagnetic shielding material obtained by laminating woven fabrics, non-woven fabrics or a woven fabric and non-woven fabric. In this case, however, in order to obtain desired electromagnetic wave shielding properties, there are the problems of an increase in the base thickness after forming the metal coating and an increase in thickness of the metal coating per se.
In addition, Patent Document 3 discloses a technology for coating a resin layer containing metal particles onto the surface of a fabric. In this technology, metal particles are coated with a resin in order to improve adhesion of a metal thin film layer, and the resulting sheet has ample flexibility. Thus, an object of the technology disclosed in Patent Document 3 is considered to be facilitating use when actually using an electromagnetic shielding sheet. However, the high level of electrical conductivity of the metal is conversely impaired by resin coating, ultimately resulting in a thick sheet that does not have high electromagnetic wave shielding properties.
Other attempts are disclosed in Patent Documents 4 and 5 in which electromagnetic wave shielding properties are attempted to be enhanced by using ultrafine fibers and utilizing the relatively high surface area obtained by carrying out metal processing on the surface of the fibers. However, in the fiber sheets disclosed in these documents, although a high-performance sheet is obtained by using ultrafine fibers, due to the weak strength of the ultrafine fiber sheet, it was unable to be used practically. Namely, although it is necessary to attach highly electrically conductive metal to polymer fibers in order to enhance electromagnetic wave shielding performance, and electroless plating by a wet method is typically used to obtain high levels of electrical conductivity and adhesiveness, in this process, the electrochemical reaction in the form of plating is repeated for multiple layers, while drying and exposure to a plating bath are also repeated many times. Consequently, the fabric itself ends up tearing or defects end up forming in the fabric during these processes, thereby preventing the obtaining of uniform processing. In addition, although it is necessary to deposit metal by placing in a vacuum chamber in the case of methods involving metal deposition, strength is unable to be imparted to the fabric at this time causing it to tear or causing defects to form in the fabric. In addition, in order to ensure reliable adhesion to the fiber surface, it is necessary to use a high vacuum and activate the surface of the fibers, and the fiber fabric was unable to withstand these processes.
In this manner, an electromagnetic shielding sheet that is effectively thin and demonstrates high performance has yet to be obtained.